This invention relates to a scanning mechanism for rapid back-and-forth mechanical scanning of an ultrasonic transducer or other like device in sensing, measuring, or analyzing a specimen or target. A "scanning mechanism" is a mechanical conveyance which moves either a specimen or a transducer back and forth along a line or in a raster of lines or other scanning pattern; it may also make adjustments in a third (focal) dimension. The transducer may be any of several kinds, including devices that emit or receive light or other electromagnetic waves or particles such as neutrons, positrons, or others, or streams of fluids, or aerosols, or sound waves, or magnetic or electric fields, or other media by which a target is examined or affected. This invention is particularly applicable to an ultrasonic transducer or transducer pair used for forming an image in an ultrasound C-SCAN inspection system, e.g., an acoustic microscope, and is described in that connection.
In a C-SCAN system a transducer bombards a target with acoustic energy, usually in a focused field. That same transducer, or another transducer, receives transmitted, refracted, or reflected signal energy from the target; that signal is then further processed electronically to extract information about phase, amplitude, time of flight, and/or other signal content. Typically, the emitted acoustic energy is a discrete pulse and the transducer is a highly focussed device; this requires maintenance of high positional accuracy and stability so that coherence of the signal with respect to the target zone examined and its relation in time can be kept valid. Acoustical frequencies used are usually in the range of one to one thousand megahertz; at the medium and higher frequencies, high resolution detail can be obtained, though limited in part by the positional accuracy of the scanning mechanism. Scanning is usually effected by moving the instrument head rather than the target, at least for the largest scan dimension, and thousands of pixels (picture elements) are derived for subsequent storage or display. To complete the scan in a reasonable time, the mechanical motion must be rapid; for the information to be valid, the scan must be quite accurate positionally.
To appreciate the requirement for scanning speed, a typical example can be considered: a reasonable target display might consist of 256.times.256=65,536 pixels. At the rate of one pixel per second, one scan would require over 18 hours. More practically, the rate must be of the order of ten thousand pixels/second. Conversely, to appreciate the requirement for positional accuracy, consider that detail of a fraction of a thousandth of an inch may be required. Any positional error of this magnitude may eradicate the required detail.
The requirements for speed and accuracy are generally antagonistic. In the typical C-SCAN machine, a raster or parallel set of lines is usually the pattern of choice, since it allows easy, direct mapping of the target. Such a pattern requires rapid back and forth acceleration of the scan mechanism and rapid motion (often a step function) in a second dimension; there is also a possible requirement for rapid, accurate motion in a third (focal) dimension. For these rapid accelerations, fairly massive objects must be impelled with substantial force. Their motions, and their reactive forces against their drive mechanisms, cause both simple and complex positional errors. Thus, reaction may cause complementary accelerations between various parts of the system, effectively shifting either the transducer or the target to an erroneous position. Stresses may distort the mechanism, redirecting the motional vectors so that positional errors appear in unexpected directions. Rotational moments of inertia may create similar errors.
The ultimate scanning speed limit for a C-SCAN type system is often determined independently of the scanning mechanism. The speed of the scanning mechanism should approach this ultimate limit as closely as practical to maximize system throughput. In one version of a reflection-mode acoustical microscope, discrete pulses sent through a coupling medium such as water (acoustic velocity 1480 meters/second) and then through a target of some solid such as aluminum (6200 m/second) traverse a total path length of about one-half inch (1.27 cm) through each medium in the round trip. This requires a round trip time-of-flight just under eleven microseconds. In such a system the scanning mechanism should scan new target areas at the rate of one pixel width each eleven microseconds.
Of course, time spent in acceleration and deceleration detracts from the maximum theoretically possible scan rate. Speeds should theoretically be linear throughout each scan line, with an abrupt reversal at an essentially infinite rate of acceleration at each scan line end. To achieve this in actuality is plainly impossible. However, maximization of the time the scanning mechanism is moving near its peak rate, and minimization of times required for reversal, are reasonable goals.
As the demand for scanning speed increases, accelerations are perforce limited to preclude positional errors of confounding magnitude. The compromise optimum is a sinusoidal plot of velocity, with velocity at a maximum only at the precise middle of the scan; at other times velocity changes at the maximum permissible rate. Extreme rigidity and massiveness of the scanning mechanism may be required, these requirements increasing with speed. The transducer mass becomes critical, and must be minimized so that its inertia is minimal compared with the machine as a whole.